Epidemics of enterically transmitted non-A, non-B hepatitis (hepatitis E) have been reported in Asia, Africa, and North America. Similar cases of sporadic hepatitis, presumed to be hepatitis E, account for up to 90% of reported hepatitis in countries where hepatitis E is endemic. Hepatitis E virus (HEV) has been implicated in fulminant hepatitis of pregnancy. This disease has a 20% fatality rate. That a viral agent was responsible for hepatitis E epidemics was first shown in 1983. On the basis of electron microscopy and molecular characterization it was proposed that HEV belongs to the calicivirus family. The goal of this project is to define the newly identified hepatitis E virus (HEV), determine the extent and pattern of its involvement in enterically transmitted hepatitis, and to develop a vaccine which prevents hepatitis E. We performed a retrospective seroepidemiologic study which showed that 16 of 17 epidemics of waterborne hepatitis in India were caused by HEV and that one was caused by a previously unrecognized agent. In addition, we showed that the age-specific antibody profile for HEV in Pune, India has not changed over a decade. We also analyzed sera from an epidemic of hepatitis E in Pakistan and obtained evidence that antibody to HEV protects humans against disease. Studies carried out in cynomolgus monkeys demonstrated that passively acquired antibody to HEV protected the animals from disease even when infection was not prevented. Most importantly, we demonstrated that active immunization with a recombinant HEV capsid protein protected cynomolgus monkeys completely against hepatitis E disease and partially against infection. We constructed a full-length cDNA clone of a Chinese strain of HEV for use in molecular studies. The recombinant ORF-3 protein of HEV was efficiently expressed in insect cells and a recombinant ORF-1 protein was efficiently expressed and secreted from yeast cells. These recombinant proteins are being studied to define the functions of the proteins. In addition they will serve as substrates in ELISA assays which are being developed to increase the sensitivity and reliability of serological assays.